Aluminum potliners are created in the smelting of aluminum metal and its alloys. They are the liners of the pots into which the molten aluminum is poured. A typical liner lasts about five years. The Pacific Northwest aluminum producers currently annually produce about 50,000-75,000 tons of spent potliners. (The production of aluminum generates about 35 kg of spent potliner per ton of metal.) Spent potliners are currently classified in the U.S. as hazardous waste. They contain significant concentrations of four soluble pollutants, namely, cyanides, fluorides, polynuclear aromatic hydrocarbons (PNAs), and heavy metals (such as lead, beryllium, and cadmium).
A treatment process for spent potliners must be technically feasible at a reasonable cost (with reasonable capital cost). It must produce disposable waste products that comply with all environmental control regulations. It must be robust to tolerate variations in the feed because spent potliners from different sources differ significantly in their makeup (that is, the process should be able to accept widely differing feedstreams rather than be limited to a particular feedstock). All spent aluminum potliners require treatment prior to landfill disposal, so aluminum producers are struggling to find an acceptable treatment process.
Reynold's proposed using a thermal treatment process involving high temperatures in a rotary kiln with the addition of sand and limestone to complex with the fluorides. Comalco proposed using a two-step process involving a calciner to complex the cyanides followed by a hydrometallurgical step to recover the fluorides. Pechiney's SPLIT process introduced ground potliner to a hot airflow vortex rotating at supersonic speeds to destroy the cyanides while reacting the fluorides with an additive to produce a disposable solid waste.
Details for two other spent potliner treatment processes are presented in:
(1) J. Bernier et al., "The LCLL Process - Spent Potlining Recycling Solution," 52nd Annual Conf. of Metallurgists, Quebec, Canada, Aug. 28-Sep. 2, 1993; and PA1 (2) R. Adrien et al., "A Process for Treatment and Recovery of Spent Potliners," Light Metals 1996, The Minerals, Metals, & Materials Soc. (1996) 1261-1263.
I incorporate these articles by reference. The "LCLL process" is Alcan's Low Caustic Leaching and Liming process which involves three steps that require the use of complicated reactors and associated transfer equipment First, the LCLL process digests (leaches) finely ground spent potlining in a dilute caustic solution at around 85.degree. C. for about one hour. Then, Alcan adds NaOH in a plugflow reactor at about 180.degree. C. and 160 psig to destroy the cyanide in the leach solution in about one additional hour of processing while producing sodium fluoride. Finally, Alcan adds more caustic (generally, lime) to the remaining fluoride liquor for a third hour of treatment in equipment comparable to aluminum smelter wet scrubbers to produce calcium fluoride and a recyclable, caustic leach solution. The LCLL process requires a significant capital expenditure for the processing equipment.
The Adrien process uses five stages to recover calcium fluoride, aluminum fluoride suitable for smelting, and a disposal, carbonaceous, solid waste from the spent potliner. Adrien leaches the crushed potliner, washes the solid residue with two acid washes using NH.sub.4 F or another fluoroacid, and, finally, uses a water wash.